This invention generally relates to apparatus for sealing conduits and more particularly relates to a foldable nozzle dam having a foldable extrusion-resistant seal or gasket for sealing conduits, such as the primary nozzles of a nuclear steam generator.
Before discussing the current state of the art, it is instructive first to briefly describe the structure and operation of a typical nuclear steam generator. In this regard, a typical nuclear steam generator generally comprises a shell and a plurality of U-shaped heat transfer tubes disposed in the shell, the U-shape of the tubes defining a first tube leg portion and a second tube leg portion interconnected by a U-bend tube portion. The shell defines an inlet plenum and an outlet plenum therein such that the first tube leg portion of each U-shaped tube is in fluid communication with the inlet plenum and the second tube leg portion of each tube is in fluid communication with the outlet plenum. Also in fluid communication with the inlet plenum is an inlet primary nozzle attached to the shell for delivering a radioactive primary fluid into the inlet plenum. Moreover, in fluid communication with the outlet plenum is an outlet primary nozzle attached to the shell for allowing the primary fluid to exit the outlet plenum and thus exit the steam generator in the manner discussed hereinbelow. In addition, the steam generator shell also includes a plurality of relatively small diameter manway openings therethrough for allowing access to the inlet and outlet plena so that maintenance can be performed in the steam generator. In this typical nuclear steam generator, each manway opening has a diameter substantially less than the inside diameters of the inlet and outlet nozzles.
During operation of the nuclear steam generator, the radioactive primary fluid, which is heated by the fission reaction of a nuclear reactor core, flows through the tubes as a nonradioactive secondary fluid of lower temperature circulates around the tubes. More specifically, the primary fluid flows from the nuclear reactor core where it is heated, through the inlet nozzle and into the inlet plenum. The primary fluid then flows into the first tube leg portion of each heat transfer tube, through the U-bend portion of each tube, out the second tube leg portion of each tube and then into the outlet plenum, whereupon it exits the outlet nozzle and the steam generator. Moreover, as the primary fluid exits the steam generator, it is returned to the nuclear reactor core to be reheated. Of course, as the primary fluid flows through the heat transfer tubes, it gives up its heat to the secondary fluid circulating around the tubes for producing steam in a manner well known in the art. Such a typical nuclear steam generator is more fully disclosed in U.S. Pat. No. 4,079,701 entitled "Steam Generator Sludge Removal System" issued Mar. 21, 1978 to Robert A. Hickman et al.
Periodically, it is necessary to shut down the nuclear reactor core for refueling. At that time, it is cost advantageous also to simultaneously perform maintenance on the steam generator. During such maintenance activities, a reactor cavity, which encloses a reactor vessel containing the reactor core, is partially drained of primary fluid to a level that is below the elevation of the inlet and outlet nozzles of the steam generator. Of course, for safety reasons the nuclear reactor core is not uncovered when the reactor cavity is partially drained of primary fluid. Thus, it will be appreciated that this process of partially draining the reactor cavity to an elevation that is below the inlet and outlet nozzles of the steam generator also drains the heat transfer tubes as well as the inlet and outlet plena of the steam generator. After the steam generator tubes and the inlet and outlet plena are drained of primary fluid, nozzle dams are inserted through the relatively small diameter manways and installed in the mouths of the inlet and outlet nozzles to block the nozzles. Once these dams are installed, the reactor cavity can be refilled with primary fluid for the refueling operation, the reactor cavity being refilled to a level above the elevation of the inlet and outlet plena. Therefore, refilling of the reactor cavity with primary fluid can be accomplished without interfering with maintenance activities being performed in the steam generator plena because the nozzle dams which block the inlet and outlet nozzles prevent the radioactive primary fluid from rising into the inlet and outlet plena. Moreover, simultaneously performing reactor refueling and steam generator maintenance activities reduces the total time the nuclear reactor core is shut down, thereby recapturing revenue that would otherwise be lost when reactor refueling and steam generator maintenance are performed in seriatim. In addition, as stated hereinabove, the manway openings have a diameter smaller than the inside diameter of the inlet and outlet nozzles. Therefore, a problem in the art is to provide a nozzle dam that is not only capable of passing through the relatively small diameter manways but also capable of being disposed across the relatively larger inside diameters of the inlet and outlet nozzles to block the nozzles.
Of course, once installed across the inlet or outlet nozzle, the nozzle dam should be fluid-tight so that primary fluid will not rise into the inlet and outlet plena to interfere with maintenance activities being performed in the steam generator. In this regard, the nozzle dam may include at least two parts sized to pass through the inlet or outlet nozzle, the two parts having a seal or gasket therebetween to seal the nozzle dam so that the nozzle dam is fluid-tight. The seal or gasket, which is intended to be clamped between the two parts for creating a seal therebetween, may have at least one aperture for passage of clamping means therethrough. However, applicants have observed that the aperture of the seal or gasket may extrude away from the clamping means when the two parts are tightly clamped together, thus enlarging the fluid flow path defined by the aperture surrounding the clamping means. This is undesirable because such enlargement of the flow path compromises the ability of the seal or gasket to perform its intended function of providing a nozzle dam that is fluid-tight. Therefore, another problem in the art is to provide a nozzle dam having a seal or gasket that resists extrusion away from such clamping means so that the nozzle dam is fluid-tight.
Steam generator nozzle dams having seals are known. One such nozzle dam is disclosed in U.S. Pat. No. 4,637,588 entitled "Non-Bolted Ringless Nozzle Dam" issued Jan. 20, 1987 in the name of John J. Wilhelm et al. and assigned to the assignee of the present invention. This patent discloses a nozzle dam having one or more seal assemblies, each seal assembly including a foldable circular seal plate encircled with an inflatable seal which is disposable in frictional engagement with the nozzle wall. However, this patent does not appear to disclose a nozzle dam having an extrusion-resistant seal or gasket.
Another nozzle dam having a seal assembly is disclosed in U.S. Pat. No. 4,671,326 entitled "Dual Seal Nozzle Dam and Alignment Means Therefore" issued Jun. 9, 1987 in the name of John J. Wilhelm et al. and assigned to the assignee of the present invention. This patent discloses a seal assembly including a foldable circular seal plate having a center section hingedly connected to two side sections. However, this patent does not appear to disclose a nozzle dam having an extrusion-resistant seal or gasket.
An extrusion-limiting seal or gasket is disclosed in U.S. Pat. No. 4,181,313 entitled "Seals And Gaskets" issued Jan. 1, 1980 in the name of Edward F. H. B. Hillier et al. According to this patent, a seal or gasket, which has at least one aperture providing a fluid passageway and which is intended in use to be clamped by clamping means between surfaces of two parts to be sealed, comprises an elastically-compressible material and a relatively-incompressible material bonded therein, the relatively-incompressible material forming an extrusion-limiting barrier which extends at least partway around the aperture. However, this patent does not appear to disclose a foldable nozzle dam having a foldable extrusion-resistant seal or gasket.
An anti-extrusion sealing device is disclosed in U.S. Pat. No. 4,468,042 entitled "Anti-Extrusion Sealing Device With Interlocked Retainer Portions" issued Aug. 28, 1984 in the name of Aaron J. Pippert et al. According to this patent, a relatively soft body, including a sealing portion, and a harder body, which serves as an anti-extrusion device for the soft body, are permanently joined together by mating mechanical interlock formations. However, this patent does not appear to disclose a foldable nozzle dam having a foldable extrusion-resistant seal or gasket.
Thus, although the above-recited patents may disclose nozzle dams and anti-extrusion seal devices, these patents do not appear to disclose a foldable nozzle dam having a foldable extrusion-resistant seal or gasket, the nozzle dam being foldable for passing through the relatively small diameter steam generator manway and being unfoldable for placement across the larger diameter of the nozzle in combination with a foldable seal or gasket attached to the nozzle dam, the seal or gasket being extrusion-resistant for providing a nozzle dam that is fluid-tight.
Therefore, what is needed is a foldable nozzle dam having a foldable extrusion-resistant seal or gasket for sealing conduits, such as the primary nozzles of a nuclear steam generator.